Unfairly handed over

Paid my account in 2019 after I was told to pay an amount of approximately R1200 due to cancelling my contract early. 3 years later, and my account is sitting with mbd attorneys cause I apparently still owe on the account. After querying with virgin active, a representative states that I had only paid in half of what was due. Don't see how this is my problem or fault as this should have been told to me 3 year's ago!!! Not my fault and I refuse to pay. Close my account!!!